Time division multiplex system



June 29, 1954 J. o. EDsoN TIME DIVISION MULTIPLEX SYSTEM 4 Sheets-Sheet1 Filed Oct. 19, 1944 Simon .351

x in sbt June 29, 1954 J. c. EDsoN TIME DIVISIONv MULTIPLEX SYSTEM FiledOct. 19V, 1944 4l Sheets-Sheet 2 M536 .nana Ok /An/VENTOR J. 0. ESON@www ATTORNEY June 29, 1954 J. o. EDSON TIME DIVISION MULTIPLEX SYSTEM 4Sheets-Sheet 3 Filed OC'. 19, 1944 .ENQ ww mit@ E mwN @12% /NVENTORJQE'DSON BV A ATTORNEY June 29, 1954 J. o. EDsoN 2,682,575

TIME DIVISION MULTIPLEX SYSTEM 0 l 4, PLATE VOLTAGE- TUBt 82 I GRIDVOLTAGE- TUEE 89 J PLATE CURRENT of' Tuse a9 /N VE N TOR J. O EDSON A TTORME Y Patented June 29, 1954 Y 2,682,575 TIME DIVISION MULTIPLEXSYSTEM James O. Edson, Great Kills, N. Y., assignor to Bell TelephoneLaboratories, Incorporated, New York, N. Y., a corporation of New YorkApplication October 19, 1944, Serial No. 559,354

This invention relates to multiplex telephony and more particularly tomultiplex systems in which the common transmission path is shared by theseveral communication channels on a time division basis.

Objects of the invention comprise the improvement of the signal-to-noiseratio in the several communication channels, diminution of interchannelcrosstalk, simplification of the transmitting multiplex equipment, andprovision for calling and alarm signals.

The invention makes use of the method of pulse position, or pulse phase,modulation according to which uniform pulses of very short durationrecurring at a rate at least twice as great as the highest signalfrequency are modulated in time position or phase in accordance with thesignal to be transmitted. For telephone communication where the speechband may be restricted to the frequency range below about 3500 cyclesper second, a pulse recurrence rate of about 8000 cycles per second anda pulse length of about one microsecond would be typical.

`In accordance with the invention multiplex operation is accomplished bydividing the period corresponding to the recurrence rate into a numberof equal smaller intervals corresponding to the desired number ofchannels together with an additional shorter interval foruse insynchronizing. Thus in the case of an eight-channel system therecurrence period of 125 microseconds would be divided into eightchannel periods of l5 microseconds each and a synchronizing period of vemicroseconds. The recurrence frequency is generated by a stable markeroscillator from the output of which there are derived a train of shortpulses, eight for each cycle, normally timed to occur at the mid-points-of the channel times, and a longer pulse for synchronizing which occursin the synchronizing period and marks the beginning of a cycle.Modulation of the pulses by the signal waves varies their positions orphases with respect to the mid-period times, the degree of modulationbeing limited so that the pulses in any particular channel do not moveoutside the channel period.

Each group of pulses including the synchronizing pulse may be called aframe, the timing of each channel pulse being controlled in relation tothe start of the frame in which it appears. Since each frame is to thisextent independent of the others, the system has some of thecharacteristics of a start-stop system.

At a receiving point the synchronizing pulses are separated from theothers by virtue of their 21 Claims. (Cl. 179-15) greater length and areemployed to generate in the receiving apparatus for each channel gatingby iiltering out all components of the pulses outside the signal range.

An important feature of the invention, which contributes greatly to thesignal-to-noise improvement, is the derivation of new pulses at thereceiver from the trailing edges of the received pulses as a first step.I have found that under practical -conditions of operation, particularlywhere the pulses are transmitted by radio as trains of high frequencywaves, the timing of the leading edges of the received pulses becomesuncertain whereas the timing of the trailing edges is not affected. Ifthe leading edges are allowed to control the processes of reception anddetection, the effect ofthe timing uncertainty is to increase the noiseaccompanying the demodulated signal. This` noise is substantiallyeliminated by controlling the processes by the trailing edges of thepulses.

Another feature of the invention lies in the arrangements providing forthe use of relaxation circuits in the generation of the positionmodulated pulses. 'Ihe use of such circuits results in a verysubstantial reduction in the number of vacuum tubes required as comparedwith other methods. It is necessary, however, that they be allowedadequate time between successive operations for their completerelaxation. To provide Vthis the several channels at the transmitter aredivided into two groups which are started at different epochs of themarker oscillator period.

These and other features of the invention and its mode of operation willbe more fully understood from the following detailed description of atypical embodiment and by reference to the accompanying drawings ofwhich:

Figs. la and lb are block diagrams illustrating the schematicarrangement of a complete multiplex system in accordance with theinvention;

Fig. 2 is a circuit diagram of the transmitting portionA of themultiplex system;

Fig. 3 is a circuit diagram of the receiving portion of the multiplexsystem; and

Fig. 4 is a series of diagrams showing the charaeter of the voltages atvarious points in the system and explanatory of the operation of theinvention.

The block diagrams of Figs. la and 1b show the arrangement of theterminal apparatus in an eight channel multiplex system embodying theinvention. Apparatus for one-way communication only is shown, Fig. larepresenting a transmitting terminal and Fig. 1b the cooperatingreceiving terminal. For two-Way communication duplicate apparatusdisposed for transmission `in the opposite direction would rbe provided,`the channels being operated as four-Wire circuits." The terminals, areshown-connected by a radio link, for which use the inventioniswelladapted, but it will be understood that the invention is not limited tocommunication Lin ,this rtransmission medium.

The method of signal transmissionis that of pulse position modulation.In accordance with fthis method', short uriidirectional pulses recur-:ring periodically at a `rate atleast twice as high -asthe highestfrequencycomponent in-the sig- 'naliare varied in'their itimesfofoccurrence, or `time-phases, in vaccordance :With-the signal cur- Arent..f'signal `frequencies :mayfbe limited toa yband be- :low about 3500cycles per. second, `a pulse fre- '..quency of 8,000 cycles'perfsecondis satisfactory.

For vtelephone communication Where the The method maybe regarded broadlyas sampling the instantaneous values-ofthe signal wave -atuniformintervals-'.8000 .times a second and sending out correspondingapulseswhich reprefsent the'values oftheisamples bythe variations in theirtiming.

.'Multiplexing iseffectedlby time division. As-

suming a pulse frequencycf A800() per second for eachchanneL therecurrenceperiod of 125 microsecondsis divided into eight vchannelperiods of `15 microseconds each, the extra interval of 5 Thepulses fromall the transmission to the receiving terminal.

For'racli'o transmissionthefpulses are converted into short trains'of'ultraehighffrequency oscillations of .corresponding ,length andtiming. At

rthe receivingterminal, these'trainsare receivedl and rectified andtherectiiied pulses are delivered to the receiving multiplexequipment.Herethey areseparated by time-division y'and directed into `theirrespective 'channel'circuits and are then converted into varyinglengthfpulses'irom'which the signals can beobtaineddirectly.

Transmitting Amultiples' In Fig. 1A the incomingfsignal lines are shownat'the left of lthedrawing and are designated channels I to 8,respectively. The Anumbering .conforms to the time order ofthe'channelsin the multiplex cycle. Channel 4Wi1l be taken as representative for'the `purpose of description. YCurrents from the incoming line-enter aiilter Ill-which serves torseparate thesignal currents .from .lowfrequency-.currents used for ringing and also to limit the :signalcurrents to a frequency band extending. fromabout 200to' 3500 cycles persecond. .Thesignalmurrents pass to amnlier .il and thence l.to.pulsemodulator I2 Jgenerate length modulated pulses Which are -timed toend normally at the mid-points of the respective channel periods andwhich vary in length within the limits of these periods under theinfluenceof the Signal currents. The abrupt endings 'of the lengthmodulated pulses are :caused to-energize a pulse generating and shapingsystem in which position modulated pulses of uniform'shape and energyare produced and delivered to the common transmission line or medium.Since 1the .pulses lnccur;serially, the `shap- 1ing'systemxmaybesharedby all channels :in common. .Howevery itis advantageousfor theprevention of interchannel 'crosstalk, to use gtwo shaping networks, onefor the odd numbered channels :and onetfor .the evennumbered channels..The two shaping :netWorkscanbe matched yWithout diinculty. so :thatithel `advantage `of univformity of :the ontgoingpulses isinot lost.

The =pulses `supplied to 1 the `:modulators are derived from a masteroscillator I3 which also Lfurnishesa lsynchrm'iizing pulse. This`oscillator operatesatagstable .frequency of 8000 cycles per -second andis ,designed -to .deliver 'a .pulsating output :.voltage ofsubstantially rectangular wave form. By ymeans of differentiatingnetworks, sshort .pulses ,alternately .positive and negative .areproducedatthe instants ,theoscillator output voltage .changes from onevalue to the other. The :negative pulses fare used to ttime the start:oizthe multiplex Cyclesandl'alsorare suppliedto`themodulators"of.channels I tot. The'positive pulses are supplieddirectly-to channels 5 to 8. The fmodulators, 4by Avirtue of theircircuit 'dereign, respond'only topositive'pulses, consequent- .ly .the'negative 4pulses must `be reversed vin sign before being deliveredtothe lowerchannel group modulators.

The :pulse Msupply .for channel :d comprises differentiatingnetwork Ill,exciter I5, in which lthe Signs 'of thepulses Aare reversed, and leadI6. These elements also supply pulses to the modulator in channel l`2.Similar elements Iii', l5' and l6"supplypulses to channels l and 3.Pulses are supplied directly'to` channels' to 8 through dilierentiatingnetwork Il. The modulator output voltage in `.channel t is delivered tothe pulse 'former i0, which is shared with the other even'numberedchannels A similar-pulse former I8 y called upon to deliverpulses of large power'in Other methods of producing the ultra-highfrequency pulses might,` of course, be used. For example, the signalpulses might be employed in an obvious manner to-unblock a path from asource of continuous oscillation to the antenna. In such case theuncertainty of the pulse duration would be largely eliminated, but theadvantage of a low duty cycle would be lost.

Signal input circuit The detail circuit arrangements of the transmittingterminal, with the `complete circuit of channel 4, are shown in Fig. 2.The incoming signal line is connected to transformer 2l in which signaland ringing currents are separated. The signal current output pathincludes the transformer secondary winding, level adjusting pad 22,low-pass filter 23 which limits the signal band to about 3,500 cyclesper second, and signal amplier 24. Resistance R1 in series with the gridof tube 24 serves to limit the output voltage of the tube in onedirection, limiting in the other direction being eiected by the tubecut-off. T-he primary winding of transformer 2l is divided and the twoparts are connected through a condenser C3 to the terminals of which areconnected leads 25, forming an output circuit for low frequency ringingcurrents. The con-r denser should be such as to offer very littleimpedance to speech frequencies but to present a substantial impedanceto 4ringing frequencies of about 20 cycles per second. Elements 2 l, 22and 23 correspond to network l0 in Fig. 1A and amplier 24 to amplifier II.

Modulator I2 of Fig. 1A comprises tubes 21, 28 and 29 and theirassociated circuits. Before proceeding with the description of thiscircuit, it will be desirable to deal with the master oscillator andpulse supply circuits.

Master oscillator The master oscillator comprises tubes 30 and 3i ofwhich 30 is the oscillator proper and 3l a wave shaping amplifier. Theoscillator is of the simple tuned grid circuit type comprising a tunedcircuit L1C1 coupled inductively to the plate circuit of the vacuum tubeandl also through a stopping condenser and gri-d leak to the grid. Thecircuit vis proportioned to provide strong feedback so that the vacuum`tube operates in class-C fashion, that is, with its plate current owingin spurts of less than half cycle duration. A pulsating output voltagecorresponding to the plate current variation is taken from cathode leadresistor 32 and applied through a condenser to the grid of tube 3l. Thegrid oi this tube swings alternately between positive and negativevoltages suiliciently great to produce alternate saturation andinterruption of its plate current. The plate current to tube 3l issupplied through a high resistance, consequently the plate potentialdrops to a low value when the current is flowing and rises to the fullvalue of the supply voltage when the current is interrupted. The

6 output voltage of the tube thus develops a substantially rectangularwave form alternating each period between a low value during the platecurrent spurt in the oscillator tube and a high value for the remainderof the period. For an oscillation frequency of 8000 cycles per second ora periodic time of 125 microseconds, a suitable adjustment of the waveform would make the low voltage interval 35 microseconds and the highvoltage interval microseconds. The divisionof these intervals can becontrolled with sufficient accuracy by adjusting the ratio of seriesclipping resistor III to the grid leak resistance II2.

It may be noted here that the on-olf method of operating tube 3l ischaracteristic of the operation of most of the tubes in the system. Itis obtained in the usual fashion by the use of high resistance platesupply circuits free from series inductances or by-passing condensers sothat they admit of very little energy storage. Heater type tubes areused throughout, the cathode heating circuits being omitted in thedrawings. Where pentodes or other multigrid tubes are indicated in thedrawings, the energizing circuits for the extrak grids, being ofconventional types, are also omitted from the showing for the sake ofclearness. In a typical system constructed in accordance with thedrawings, a plate supply voltage of 300 was used.

Exciter The output voltage of tube 3l is applied to the grid of excitertube 26 through a differentiating network comprising resistance R2 andcondenser C2, respectively, 1 megohm and 10 micro-microfarads.Resistance R2 being connected to the positive terminal of the platesupply voltage normally holds the grid at a potential just slightlyabove the cathode potential.- When the plate of tube 3l suddenly dropsto its lower potential, the. change is immediately transmitted throughcondenser C2 to the grid of tube 26 drivingthe grid to a negativepotential sufficient to block the plate circuit. As the small capacityof the conrtial and unblocks the plate circuit. At the later time whenthe applied voltage suddenly rises, a corresponding positive pulse tendsto appear at the grid 0f tube 26, but this pulse is very much reduced bythe grid conductance. Since the tube is at this time drawing suiiicientplate current to reduce its plate voltage to a very small value, theweak positive pulse on the grid has negligible effect on the outputvoltage.

Exciter tube 26 normally draws plate current through resistor Ra whichis also connected to the grid of the succeeding tube. This resistor hasa resistance of l megohm and, together with the additional seriesresistance R9, holds the plate potential of tube 28, at a low value.Resistance Rg, which may have a value of about 120,000 ohms, togetherwith R3 forms a voltage divider providing a suitable grid potential fortube 2l' in the relaxed condition of the circuit. rlhe mo mentarynegative pulse applied tothe grid of tube 26 from the differentiatingnetwork results in a corresponding positive pulse at its plate and alsoon the grid of tube 2l.

M o-dulator As already indicated, the modulator I2 of Fig. 1A comprisestubes 21, 28` and 29, the first two constituting a relaxation circuit,or one-shot .multivibraton characterized by a single -stable conditionand the last beingan output tube. The

relaxation circuit tubes 21 and 28 are coupled in one direction byresistor R7, condenser C4, to-

in the opposite direction by the common cathode lead resistor Re. Thecouplings are so proportioned that the circuit does not oscillate andthat in its normal stable condition tube'28 `conducts and tube 21 isblocked. In this condition the-grid yof tube 21 is held, by means ofresistor Ra'and the circuit through tube 26, at a potentialsubstantially lower than that of its cathode.

When the grid of tube 21 is driven momentarily positive by the pulsefrom the exciter tube 26, its plate circuit becomes conductive and anegative pulse is transmitted through condenser Ci to the grid of tube2S. By virtue of the coupling in the reversed direction through cathoderesistor R8, the reduction of the plate current in tube 23 due to thenegative-pulse proceeds very rapidly and almost instantaneously' bringsabout a complete transfer of the plate current to tube 21. In thiscondition, which may be termed the strained condition, the platecurrentin tube 21 is less than that which had previously existed in tube 28 andis such as to hold the cathode of that tube at a potential slightlyhigher than the grid potential. The condition is maintained until thegrid potential of tube 28 is restored by the charging of condenser C4through resistances Ri and Rs to a value only slightly negative withrespect to the cathode whereupon the circuit returns very rapidly to itsrelaxed condition.

The time interval between `the exciting pulse and the instant ofrelaxation is called the relaxation time. The principal circuit elementscontrolling this interval are capacity Ci, resistance R7, and resistanceRi, the latter resistance being very large compared with the otherresistances in the circuit. These elements are proportioned for eachchannel so that the relaxation takes place normally at the middle of thecorresponding channel period. Ordinarily it is satisfactory to make theresistance R4 the same `for all channels and to control the relaxationtimes by varying the capacity C4. The resistance may conveniently beabout 3.3 megohms and the capacities may range from about 1D to about15G-micromicrofarads. Adjustment or' the 'relaxation time to make up forvariations in other constants is accomplished by adjusting Ri whichcontrols the change of voltage applied to grid of tube 28 in thestrained condition and so varies the time.

The character of the voltage variations in `the parts of the system sofar described are illustrated by the diagrams a to d in Fig. d. Thevarious lines represent the voltages as functions of time during onecomplete cycle of the master `oscillation, that is, for one completemultiplex cycle. The light vertical lines indicate the division of theperiod into eight channel periods Vand one shorter synchronizing period.Graph a represents the oscillator output voltage at the-plate of tube3i. The multiplex cycle startsat the'instant this voltage drops from itshigh'to its low value as indicated in the drawing. The two parts of thewave last and 90 microseconds, respectively. Graph b shows the voltagepulses on the grid of exciter tube 26 resulting from the action of thediierentiating network. The positive pulse occurring whenthe oscillatorvoltage suddenly increases is curbed somewhat by the flow of gridcurrent. The fullline curve in c shows the-.starting pulse -on'the gridof tube 21 8 ofthe relaxation-circuit corresponding to the negativepulse on the exciter grid but reversed in sign. The slight eiect of thesucceeding positive pulse is also indicated. The dotted line shows thecommon cathode potential of the relaxation lcircuit tubes in relation tothe grid potential of tube 21. Graph d shows the potential variation atthe grid of tube 28 and its recovery to its origi- .nal yvalue at themidpoint of channel d period.

This graph represents also the grid potential of output tube 2S. Thegrid voltage starts at about the same potential as the cathodes in therelaxed condition of the circuit. Upon the arrival of the exciting pulsefollowed by the sudden transfer of the plate current, it drops sharplyby the same amount as the drop of the plate potential in tube 21. Thevoltage begins to recover immediately and continues to do so at asubstantially uniform rate until it'reaches a value about equal to thediminished cathode potential. At this point relaxation takes place andthe potential rises rapidly toits original value. Ihis is indicated bythe small step at the end of the sloping part of the graph.

The length of the relaxation time depends not only on the time constantR404 but also on the effective value of the voltagecharging condenserC4. It can be controlled to some extent by the adjustment of resistanceR7 since this determines the initial swing of the plate voltage in tube21 and henceof the grid of'tube 28. This adjustment may be used for thenal centering of the channel pulses and for readjustment when a. newtube is required.

To bring about the modulation of the relaxation time by the signalvoltage, resistance R4 is connected to the junction point of tworesistances R5 and R6 which together form the load resistance'of signalamplifier tube 24. In this way apart of the signal voltage issuperimposed on the charging voltage and the rate of charging isincreased or decreased accordingly. The signal voltage is effective onlyduring the charging time and, since this is only a'fraction of therecurrence period, the voltage may be regarded as being substantiallyconstant and as being representative of the instantaneous-value of thesignal.

Output tube 29 is so arranged that its plate circuit is blocked at thesame time as that of tube 28 and is suddenly unblocked at the instant ofrelaxation, the sudden unblocking being used to create the positionmodulated channel pulse. This tube serves also to stabilize the timingof thevrelaxation circuit. By means of potential dividing resistances 34and 35, the latter of which is shunted by a relatively large capacity,the

cathode of tube 29 is held at a xed potential slightly lower than thatat which it is desired to maintain the cathodes of the modulator tubesin the relaxed condition of the circuit. The grid -of tube 23 thus drawsno current during the period of relaxation, all of the current beingdserves to stabilize the cathode potential of the modulator tubes,particularly against theenects of tubevarations such as occur due toaging.

The modulator'cathode potential in therelaxed condition is equal to theproduct of the plate current in tu-be 28 and the resistance of Ra. If,due to aging, the plate current should tend to diminish, then thecathode potential would likewise tend to diminish. Any such variationwould, however, be accompanied by a change in the potential dilferencebetween the cathode and the fixed potential grid of such character as tobring about a compensating change in the plate current. It is also to benoted that the absence of grid current in tube 28 eliminates any effectthat variations thereof might otherwise have on the cathode potential.

The extent of the initial negative swing of the grid potential in tube28, which is determined by the plate current drop in R7 is stabilized bythe feedback eiect of cathode lead resistor Re and depends principallyon the potential maintained on the grid of tube 21 by the voltagedivider com.- prising resistances R3 and R9, and the plate circuit oftube 26.

By virtue of the stabilizing arrangements described above substantialdiminution of variations in the relaxation time 'of the modulator havebeen achieved. The use of the auxiliary output tube 29, to stabilize thegrid and cathode potentials of tube 28 has been found in practice toreduce the variation incident to changing tubes to about one half theamount that occurred when the plate current to tube 28 was controlledonly y by the plate feed resistance.

Pulse former The pulse forming system shown at I8 in Fig. 1A comprisestubes 36 and 3'! and their associated circuits. The grid of tube 36 isconnected to the plate of tube 29 through capacity C5 and to groundthrough an inductance L2. This inductance resonates with thecapacitances of the tube and the wiring connected to the grid, indicatedby the dotted lines, at 500,000 cycles per second and, with thesecapacities, constitutes the circuit in which the final pulses areformed. The pulse formations take place as follows: After the platecircuit of tube 29 has been blocked the plate potential builds up fairlyquickly as C is charged through the plate supply resistance so thatbefore relaxation occurs Cs becomes charged almost to the supplyvoltage. At the instant of relaxation tube 29 suddenly becomesconductive permitting C5 to discharge through its plate circuit and sotodevelop an exciting pulse at the terminals of the tuned circuit. Theoscillation takes place at the frequency of one-half million cycles persecond and during. the rst half cycle drives the grid of tube 36negative. |The second half cycle tends to drive the grid positive butthe flow of grid current absorbs the energy and prevents furtheroscillation. The resultant pulse is therefore substantially a singlesinusoidal half Wave of one microsecond duration. The pulse is repeatedas an amplied positive pulse `on the grid of tube 3l sufciently strongto overload the tube and effect a squaring of the pulse shape. The platevoltage variation in tube 29 is shown in graph e of Fig. 4, the risingportion oi which illustrates the effect of the charging current incondenser C5, and the pulse on the grid of tube 36 is shown in graph f.Since this tube is shared in common by all of the even-numberedchannels, additional pulses derived from the other channels will alsoappear. These areshown dotted in the gure. The additional pulses wouldalso react on the plate potential of tube 29, but this effect is notindicated in the graph e.

- The connection of the pulse forming system to the other even-numberedchannels is made through lead 38, this lead going through othercondensers similar to C5 to the plates of the modulator output tubescorresponding to tube 29. The forming system for the odd-numberedchannels is connected to the output side of tube 3'? by lead 39. Fromthis point the circuit goes on to the radio transmitter, shown at i9 inFig. 1, through leads 40.

The use of duplicate pulse forming systems for the odd numbered and theeven numbered channels instead of a single forming system for allchannels results in a substantial reduction of the interchannelcrosstalk. Such crosstalk may be produced in the forming system when atransient following the production of one pulse endures long enough toaiect the timing of the subsequent pulse corresponding to a differentchannel. By using separate forming systems for the alternate pulses thetime interval between successive pulses in each former is doubled andmore complete disappearance of transients in the pulse intervals isensured. Shorter channel periods than would otherwise be permissible canthus be used and the number of channels for a given recurrence period isincreased.

VUpper channel group modulators The relaxation circuits for the upperchannel group, 5 to 8, inclusive, are started together at the laterpoint in the cycle when the oscillator voltage suddenly rises to itshigher value. The division of the channels into the two groups and thestarting of the relaxation circuits of the two groups at different timesensures suflicient time for each circuit to be restored fully to itsstable I condition before it is restarted for the next cycle. Since thecircuits are started by positive pulses and do not respond to negativepulses, the posi tive pulses produced by differentiation when theoscillator voltage suddenly rises may be used directly Without the helpof an exciter tube. The relaxation circuit for one of the odd-numberedchannels, for example channel 5, and its connection to the masteroscillator are shown in the lower right hand portion of Fig. 2. Thecircuit comprises tubes il and 42 and is substantially the same as thecircuit comprising tubes 2'! and 28 except that the grid of the firsttube is coupled to the oscillator directly through the differentiatingnetwork CsRu. Resistance R11, of about 120,000 ohms, forms a potentialdivider with Rio, by means of which the steady value ci the gridpotential is fixed. This resistance takes the place of R9 and theexciter tube plate circuit of channel 4 in so far as the function ofdetermining the grid potential is concerned. The grid of tube 132 isconnected by leads i3- and dal, respectively, to the load resistance ofthe signal amplier and the grid of the output tube as in channel it. Thevariation of the grid potential in tube 42 during the recurrence cycleis illustrated by graph gin Fig. 4.

Marker generator Synchronizing pulses are generated at the start of eachmultiplex cycle by the marker generator comprising tubes 45 and ri.These pulses are distinguished from the signal pulsesYY by their Thevalues of Riz tion of the desired length. The interruption resuits in apositive rectangular vpulse at` the plate of the tube which is repeatedin." the output of tube it as a corresponding negative pulse. Thesepulses are conveyed by lead 4l to the common input circuit of the radiotransmitter. The whole array of pulses appearingat this point isillustrated by graph h of Fig. 4.

Ringing The system provides for calling-by the use of ordinary ZO-cycleringing currents in the connected wire lines. To transmit a call over` amultiplex channel the ringing currents are caused to interrupt thesignal pulses in that channel, thereby producing a signal at thereceiving terminal in a manner described later. The ringing currents,separated from the signal currents by condenser C3 are transmitted overleads 25 to a rectifier 48th-e output current from which operates relay49 to break-the cathode connection of the relaxation circuit and stopthe generation of the signal pulses.

Receiving multiplex The schematic arrangement of the circuit is shown inthe block diagram, Fig. lb. High frequency pulse trains received byradio receiver 5e are rectified and the rectied pulses after suitableamplieation are delivered to common ampliiier 5l which forms the inputstage of the multiplex equipment. At the output side of arnpliiier 5lthe circuit divides. One branch goes to a second amplier 5G and thenceto eight different channel conductors through high resistances such as5l leading tok individual pulse converters such as 55. While theconductors are marked in the drawing with their respective channelnumbers and the pulse converters are individual to the respectivechannels, it will be understood that all pulses appear in each channelat this point. The second branch goes to marker selector in which thesynchronizing pulses are The signal is nally're-covered by passing theVlength modulated pulses through low-pass filters,

such as 59, which cut off at about 3500 cycles per4 second, and thenthrough a signal amplier SB to the output line Si.

A second output circuit of the pulse converter delivers the convertedpulses to a rectifier G2, the rectified output of which is supplied aringing relay circuit 63. present the ringing circuit is heldinoperative, but on the disappearance of these pulses it 0perates andsend an appropriate ringing signal over the outgoing line. outgoing lineis indicated in a conventional manner.

It is desirable to indicate failure to receive the marker pulses, sincesuch failure wouldv usually, signify a breakdown at'some point in thesystem. For this purpose the marker pulses from the out'- put of themarker selector are supplied through a separate amplifier 64 to a markeralarm cir-'- cuit 65. The absence of pulses results in the operation ofthe alarm circuit to display a suitable warning signal. At the same timethe ringing circuits or" theseveral channels are disabled by aninterlocking connection through lead 5G so that failure of the markerpulses will not give rise to false calling signals in. the connectedvcircuits.

The circuits of the receiving multiplex are shown in detail in Fig. 3.This includes the apparatus for a single channel, channel 4, togetherith the apparatus common to al1 channels.

Common signal amplifiers Rectified and amplied pulses from the radioreceiver enterthe circuit on lead 66 and are apseparated from thechannel pulses by virtue of their greater length and are passed on toamplifier E3 and thence to a square wave generator 5t.

The square wave generator is a simple multivibrator type of oscillator,the oscillations of which are controlled and synchronized by theamplified marker pulses. Its Waveform is nearly symmetrical, dividingthe period intotwo almost equal parts. Pulses derived from the squarewave are used to start a series of gate pulse generators such as 55, thepurpose of these generators being to provide voltage pulses rectangularin form and coincident and about coextensive with their respectivechannel periods. The output circuits of the gate generators areconnected to the corresponding signal pulse circuits branching fromamplifier so that the gate pulses are superimposed on the signal pulsesat the inputs oi the several pulse converters.

The pulse converters, such as 58, are relaxation circuits similar tothose usedin the channel modulators of the transmitter. They arearranged to be started by positive pulses and are so biased that theycan be started'only when a gate pulse and a signal pulse are presentsimultaneously in their-.input circuits.. Theyv areA also arranged torelax at the end of thegate pulse. Consequently they operate to producepulses which start with the occurrence ofthe appropriate signal pulsesand stop attheendofthe gate pulses and are therefore. modulated. inlength according tov the signal.

plied to the grid of commonamplier 6l through an input networkcomprising low resistance 68, which matches the line from the radioreceiver, blocking condenser $8 and grid leak resistance 10. The pulsesat this point are of positive polarity. They are repeated at the outputof tube 8l' as negative pulses of increased amplitude and, after passingthrough blocking condenserV i l, divide into two paths one followingconductorv 'i2 to the marker selector and the other passing throughswitch Si to the grid of pulse amplier 13.

Amplifier 73 and its associated circuits provide for operating the pulsedemodulating circuits from the trailing edges of the received pulses. Asalready pointed out, the timing of the trailing edges is generally quitedenite 'when a pulse excited high frequency oscillator is used at theradio transmitter, while the timing of the leading edge is somewhatuncertain and subject to jitter To avoid the eiects of this uncertaintytube 13 is used'to generate new pulses or" substantially Xed size andshape from the trailing edges of the received pulses. Its grid is heldat a slightly positive potential relatively to its cathode and uponreceiving a pulse from the preceding tube is driven sufciently negativeto block the plate circuit. The resulting positive pulse in the platepotential is differentiatedv by the action of dierentiating networkCaRia whereby two pulses are produced at grid of the succeeding tube,lll, one a positive pulse at the start of the applied pulse and thesecond a negative pulse at the end. The rst pulse has little effect atthe output of tube 74, since the grid of that tube has a positive bias,but thev second pulse drives the grid negative and produces a.l

substantial momentary rise in the plate po- So long as signal pulsesvare" The switching of the pulses.

tential. From .the output ,of tube 14 the circuit through a plurality ofhigh resistances such as 51 to the individual channel demodulators.

The two switches S1 and Si in the input and output of tube 'F3respectively permit that tube to be removed from the circuit` ifdesired, by moving both to their upper contacts. When that is done thepulses are simply repeated through tube 'i4 and appear in the outputthereof as positive pulses coincident with the input This connection maybe used where the radio transmitter does not produce any uncertainty inthe timing of the leading edge of the pulse.

Marker pulse selector Negative pulses from the output of tube 61 aresupplied through lead 12 to the grid of marker selector tube l5. Platecurrent is supplied through high resistance R14 and the plate is shuntedto ground through a timing condenser C9. During the intervals betweenpulses the grid is held at a slightly positive potential and the platepotential is very low. When a negative pulse is applied to the grid theplate circuit is interrupted and condenser C9 begins to charge andcontinues to charge until the pulse ends. Resistance R14 and condenserC9 are chosen to provide a relatively large time constant so that therate of charging remains nearly constant in the time intervals involved.The Voltage to which condenser C9 becomes charged is thereforeproportional to the pulse length, consequently the relatively longmarker pulses will produce voltage several times as great as thoseproduced by the short signal pulses.

The voltage pulses in condenser C9 are applied to the grid of tube 76,the cathode of which is held by a potential divider, as shown, at apotential which is positive relative to the normal value or" the gridpotential. The bias thus produced is sucient to block the plate circuitand to prevent the flow of space current except in response to thelarger voltages produced by the marker pulses. These produce negativepulses' in the plate potential of tube 16 which are transmitted.

through switch kS52 and differentiating network CioRis to the grid oftube 1l.l Each pulse from tube 76 produces two pulses on the grid oftube 1l, first a negative pulse and then a' positive pulse, the latterbeing coincident with the trail- SQ'LLaTe wa/UC geneatOT 16. In thiscondition the starting of the multivibrator cycle takes place veryshortly after the appearance of the leading edge of the marker pulse andis timed in relation thereto.

'I'he square wave generator serves to provide two points in themultiplex cycle from which the subsequent operations in the process ofdemodulation can be started and so permits the channels to be handled intwo groups as in the transmitting multiplex. The iirst point is thestart 0f the cycle which is coincident with the start of the timedevoted to the signal periods., At this instant the multivibratorvoltages suddenly reverse under the impact of the synchronizing pulsefrom tube l' and this is coincident with the start of channel l. Thesecond is the time at which tube T9 becomes' conducting and tube i8 iscut oil". This time is controllable by adjusting resistance H3. It isadjusted to coincide with the start of the time assigned to channel 5.The gate pulse for channel 5 is generated by a differentiating circuitconnected to the plate of tube i9. These are the conditions for trailingedge operation. For leading edge operation some slight readjustment ofthe multivibrator would be necessary and the network generating gate forchannel l would be slightly different.

The voltage variations in the common portions of the circuit so fardescribed are illustrated by graphs i to n, inclusive, of Fig. 4. Theseref-er to leading edge operation. in graph m the dotted line representsthe grid potential at which tube 'i6 just begins to conduct. Otherwisethe diagrams with their legends are believed to be selfexplanatory. Ingraph t the dotted lines indicate possibl-e variation in pulse lengthdue to Variation in time of occurrence of pulse for channel d.

' Gate pulse generation li the gate pulsegenerator comprises tubes Sii,

The plate of tube 'Il is connected through switch S2 to the plate oftube 18 which together with tube 19 constitutes a multivibrator circuitof conventional type generating a nearly symmetrical wave ofsubstantially rectangular form. It is synchronized by the negativepulses produced at the plate of tube 11 and to facilitate this, it isdesigned to oscillate at a frequency slightly lower than the frequencyof the synchronizing pulses, namely, 8000 cycles per second. The startof the multivibrator cycle is thus timed by the trailing edge of thereceived marker pulse and is largely free from jitter.

By means of switches S2 and Sz tube 'il and its diierentiating circuitcan be out out of circuit in which case the multivibrator is controlleddirectly by the negative pulses at the platenof tube 8i and 32 and theirassociated circuits. '.i'he operation of the gate generator for thischannel is started at the instant the grid of tube 'i9 of the squarewave generator swings to a negative potential, that is, at 'the end ofthe marker pulse. Since the iinal gate pulse has to coincide with thechannel time for channel its start must be appropriately delayed. Forthis purpose tube 86 and its output network R18, C15 are arranged tooperate as a sweep voltage generator providing a voltage on the grid oftube iii which blocks the plate circuit of that tube at the start of thevoltage sweep and unblocks it at the beginning of the channel time.,

The plate circuit of tube te is blocked and unblocked in synchronismwith the square wave, being blocked duringV the tiinethat the grid oftube T9 is negative. The plate potential is very low during theunblocked period, but starts to rise at the instant of blockingcontinues to rise as condenser C15 charges. Tube Si is normally blockedby virtue of a positive bias on its cath- Ode, derived from a potentialdivider as shown, and does not become conductive until the voltage onC15 rises to a suitable value, By proper proportioning of Ria and C15this is made to take place at the start of the channel time for channel4. ITube Si continues in its unblocked condition so long as the grids oftubes TS and sweaters.

- l' retain theirl negative polarity; that/is` untilthe endof thefirsthalfof the squarewave cycle. At thatinstant the tube is restoredtoV its-normal blocked condition.

At the moment tube 8l is unblocked, that is, at the start of-the fourthchannel period, its plate potential drops sharply and thisidrop istransferred to the grid of tube 82 through condenser C11 driving itsufficiently negative to interrupt the plate current. The time constantofthe circuit comprising C11 and Ris is adjusted to restore the gridpotential to its unblocking value at the end of the channel period. Atthe start of the blocking interval the plate of tube 82, which is fedthrough high resistance 5l from the plate of signal amplier lil;increasessharply providing a gate pulse which lasts throughout thechannel period.

The successive stages in the formation of the gate pulse are illustratedby the graphs p to s of Fig. 4'. In graph q' the dotted line indicatesthe Xed-biag to be overcome by the grid'voltage in tube 8l before thetube becomes conductive. With respect to graph it may be notedthat thevoltage required to cut on" the plate current is small in comparison tothe peakyoltage of the pulse. Graph s shows the gate pulse with thefourth channel signal pulse superimposed on it and also the signalpulses of the other channels.

The grid-of tube 19 also controls the gate pulse generators for channels2 and 3 overk lead 83. These generators are similar to the generatorinchannel 4, differing only inthe constants of the sweep circuitscorresponding to C10, R15, which are proportioned to produce theappropriate time delays.v

Since the rst channelperiod starts at the moment the grid of tube I9 isdriven negative, no delay is necessary and a simpler gate generator maybe used. The simpliied generator comprises tube Sfa the grid of which isconnected to the plate of tube 'l through timing circuit C12, R17, highresistance 85 and lead 84. The plate of tube 'i8 drops in potential at atime slightly too early for channel l. This drop is transferred to thegrid of tube 8d delayed slightly by seriesy resistor 85 Working into thetube capacity, driving it negative and blocking the plate circuit. Thegrid potential is restored by charging C12 through Rm and unbloclrs-thetube at the end l of the channel period.

The gate pulse generator for channel 5 is similar to that for channel lexcept thatresistor 85 is not needed. It is controlled from the plate oftube l over lead S1. The plate potential of this tube drops sharply atthe beginning of the fifth channel period, consequently no delayis'needed. The generators for channels E, 1 and 8 are similar to that inchannel 4 and are controlled over lead 538 from the grid of tube l whichalso is driven negative at the start of the fifth channel period.

The division oi the gate generatorsinto two groups started at diierentpoints in the multiplex cycle ensures that the various timingand sweepcircuits all have ample time to be restoredto their normal conditionsbetween successive operations.

Pulse converter The pulse converter for channel 4 comprises tubes 8Sandll -which with their associated circuits constitute a relaxationcircuit orone-shot multivibrator of the same type as is used in thetransmitting multiplex. The converters for the other channels are of thesame type- In the relaxed'condition of the circuittube 90 con- 1`6`ductive and tubel isr blocked. The cathodeofV the latter` tube is heldata positive. potential .by' the flowV of theplate `current inztubeflthrough: cathode leadlresistor. 9J'. The'. gridl isV connected to theplate of gategenerator tube 82 and to: the plateof signal pulseiampliier'M through resistor 5l and is normally at a very low posit-ivepotential. The effective grid bias issuch that it cannot be overcomebyeither thegate pulse or a signal pulse` alone, but requires the sum'of:the two. The gate pulse thus-holds thecircuit ina'prepared conditionduring lthe channel periodrsothat it' is selec'- tively operated bytheproper channelpulse. TheA variation of the voltage on the grid of tubei291 isy shown by graphs of Figi. 4:

The simultaneouspresence. ofthe gate pulse and a signal pulse atthe--grid'of Vtubel-causes the sudden transfer of then plate currenttothat tube from tube 98. The circuit is so adjusted that it does notreturnto its relaxed condition immediately the signal pulse has` passed,buty is held in its strained ccnditionuntil the end of the gate puse. Tothis end the timeconstant of resistor l l5 'and-condenser.' l lll :ismadelong comparedtoA the time. assigned to aV channel. When currentflows to plate of tube 89 the grid of tube 98 is driven negative cuttingon the plate current and allowing the cathodepotentialto dropsuiilciently to ensure that tube S9 remains conductive until the gatepulse has ended. When the gate pulse ends tube 82 is driven to cut offand tube 90 becomes conductive.

The action of the circuit-is such'that the plate current in tube 8dflows in pulses which last from the appearance of the signal-pulse untilthe end of thegate pulse andrwhichzare therefore modulated in lengthwiththe leading-edges varying inaccordance with the transmitted signal.The character of the pulses is shown by graph t of Fig. l the dottedlines indicating the range of variation of the pulse length.

Signal output circuit The varying length pulsescontain the signal whichis easily recoverediby' passing the: pulses through a lo :Jpass :Filterand amplifying the iiltered currents. A pulsing Voltage is obtained`from' a tap in the load resistance 52. for tube 83- andr'is led throughblocking, condenser 93 to 'lows pass iilter ed"Whiohimayrhaveaout-ofi ataboutv 3590 cycles per second for speech signals. The. ltered signal istransmitted. through potentiometer t5` to audio amplifier S5v and thencethrough transformer el fto lineilSfWhich maylead to a conventionaltelephone sr-Jitchboard.v

Ringing and alarm circuits 4C13 suicient'to bloclcthe platecircuitoitubel 09..

When the pulses disappear.tliebiaskinfthis:tube is removed and thefflowof platelcurrent causes relay lill to operate and connect asourceofringeingcurrentl2 tothe outgoing line.` Tolensurer.

1 its prompt operation in the absence of A pulses; theV grid of tube isbiased to a slightly positive fixed potential by potential divider |03,|04. As a further means for ensuring reliable operation of relay |0|,the output of tube |00 is supplemented by current from tube 9E. For thispurpose resistor H is provided in the cathode lead of tube itil and thepotential drop across this resistor is applied t0 the grid of tube 96 byway of potentiometer 95. Accordingly when tube |00 becomes conductive,the grid of tube 96 receives an additional positive bias which increasesits plate current.

To indicate a failure to receive the marker pulses a separate alarmcircuit is provided. Pulses from condenser C'g in the marker selectorcircuit are applied to the grid of tube |05 which is biased sufcientlyto make it responsive only to the larger amplitude pulses developed bythe received marker pulses. 'I'hese pulses produce correspondingnegative pulses at the plate of the tube which pass through shuntedrectifier I to condenser C14. The negative voltage built up in condenserC14 is applied to the grid of tube |01, blocking the flow of platecurrent and holding relay |08 unoperated. The disappearance of themarker pulses results in the operation of relay les and the closing of acircuit containing a suitable alarm device |09.

Since the failure of the marker pulses would generally be accompanied byfailure of all pulses, it would result in the operation of all of theringing relays and the transmission of false calling signals over all ofthe connected lines. To avoid this the current to the ringing relays andtubes is supplied through a contact |||l of the marker alarm relay. Thiscontact is opened when the alarm relay operates and operation of theringing relays is prevented.

What is claimed is:

1. The method of multiplex telephony which comprises producingsuccessive frames of pulses each frame comprising an initial pulse ofrelatively long duration and a succession of short uniform signalpulses, one for each speech channel, following at normally equal timeintervals, the frames of pulses recurring periodically at a ratesubstantially higher than the highest speech frequency to betransmitted, modulating the time positions of the signal pulsescorresponding to the several channels by separate speech signals, f

receiving the pulses after transmission through a commonmedium,separating the longer initial pulses, producing therefrom a plurality ofgating pulses at successive times corresponding to the several channelperiods, and combining the received signal pulses with the respectivelycorresponding gate pulses to separate and detect the signals in thedifferent channels.

2. The method of multiplex telephony which comprises producingsuccessive frames of pulses, each frame comprising an initial pulse ofrelatively long duration and a suggestion of short uniform signalpulses, one for each speech channel, following at normally equal timeintervals, the frames of pulses recurring periodically at a ratesubstantially higher than the highest speech frequency to betransmitted, modulating the time positions of the signal pulsescorresponding to 18 timed by said trailing edges, and combining said newpulses with the respective gate pulses to separate and detect thesignals of the different channels.

3. In a time division multiplex telephone system in which signals aretransmitted as time modulations of uniform short pulses recurringperiodically at a rate substantially greater than the highest signalfrequency employed, sending means comprising circuits defining aplurality of channels, a relaxation circuit individual to each channel,means for deriving short pulses from said relaxation circuits at theirinstants of relaxation, said relaxation circuits operating cyclically toproduce a sequence of channel pulses in each recurrence period, a sourceof oscillations the frequency of which determines the pulse recurrenceperiod, and means for controlling the starting of the relaxationcircuits of one group of contiguous channels at one point in each cycleof oscillations from said source, and means for controlling thestarting. of the relaxation circuits ofanother group of contiguouschannels at a later point in each cycle of oscillations from saidsource.

4. In a time division multiplex telephone system in which signals aretransmitted as time or phase modulations of periodically recurrent shortpulses, sending means comprising circuits deiining a plurality ofmessage channels, pulse generators comprising relaxation circuitsindividual to each channel operating cyclically to produce a sequence ofchannel pulses in each recurrence period, a source of oscillations thefrequency of which determines the recurrence period, means forcontrolling the starting of the relaxation circuits of one group ofcontiguous channels at one I point in each cycle of oscillations fromsaid source,

the several channels by separate speech signals,

receiving the pulses after transmission through a common medium,separating the longer initial pulses, producing therefrom a plurality ofgating pulses at successive times corresponding to Athe several channelperiods, deriving from the trailing edges oi the received signal pulsesnew pulses and means for controlling the starting of the relaxationcircuits of another group of contiguous channels at a later point ineach cycle of oscillations from said source, speech input circuitsindividual to each channel, and means for varying the relaxation timesof said relaxation circuits in accordance with speech currents wherebythe time positions of the signal pulses are similarly varied.

5. In a time division multiplex system in which signals are transmittedas time or phase modulations of uniform short pulses, transmitting meanscomprising circuits dening a plurality of speech signal channels,modulated pulse generators individual to said channel, said generatorsoperating in succession during each multiplex period, a commontransmission path, a pair of amplitude limiting circuits coupled attheir output to said common path, said limiting circuits being adjustedto transmit only voltages greater than a nite fixed value, and circuitsso coupling said signal channels to the input terminals of said limitersthat each limiting circuit receives only alternately generated pulses.

6. In a time division multiplex system in which signals are transmittedas time or phase modulations of uniform short impulses, transmittingmeans comprising circuits deiining a plurality of signal channels,generators operating in succession during each multiplex period toproduce a sequence of exciting impulses normally uniformly spaced intime, means for modulating the timing of the exciting pulses inaccordance with signal currents in the respective channels circuits, acommon transmission path, a pair of pulse forming circuits coupled attheir output terminals to said common path, said forming circuitsproducing in response to exciting pulses impressed upon their inputterminals, short pulses of uniform length and substantially rectangularwave form, circuits coupling those of said` generators producing pulsesof odd order in the sequence to one of said forming circuits, andcircuits coupling the others of said generators to the other formingcircuit, whereby each of said pulse forming circuits receives onlyalternate exciting pulses.

7. in a time division multiplex telephone system in which signals aretransmitted in a common path as time modulations of uniform shortpulses, and in which the signal pulses for the several channels aretransmitted in sequences separated by synchronizing pulses, receivingmeans ccmprising a plurality of signal circuits, one for each channel,and an additional synchronizing circuit coupled to said common path,relaxation circuits included in said signal circuits and connected toreceive signal pulses, means in said synchronizing circuit for selectingthe synchronizing pulses, pulse generating means in said signal circuitsoperating under the control of the selected synchronizing pulses togenerate gating pulses substantially coincident and coextensive with therespective channel periods, and connections for impressing the gatingpulses upon the relaxation circuits together with the correspondingsignal pulses, whereby the relaxation circuits are caused to generatelength modulated pulses lasting fromv the time of occurrence of thesignal pulse to the end of the gate pulse.

8. In a time division multiplex telephone system in which signals aretransmitted in a common path as time modulations of periodicallyrecurrent short pulses and in Which the signal pulses for the severalchannels are transmitted in sequences separated by longer synchronizingpulses, the method of reception which comprises selectively receivingthe synchronizing pulses, deriving new synchronizing pulses from thetrailing edges of the selected pulses, producing under the control ofthe derived synchronizing pulses a succession of gate pulses coextensiveand coincident with the several channel periods, deriving new signalpulses from the trailing edges of the received pulses, and combining thederived signal pu es with the respectivelycorresponding gate pulses toseparate and detect the signals in the diierent channels.

9. A multichannel transmitting system comprising a source of pulses,retardation means to retard by diiferent amounts the energy of saidpulses thereby producing a plurality of differently timed trains ofpulses from said source, each train representing a channel forcommunication, means producing a train of unretarded pulses or" longerduration than the pulses of the other trains for use as synchronizingpulses, means to 'modulate the pulses of the retarded trains in timerelative to the timing of said synchronizing pulses according toinstantaneous values of signal intelligence, and transmitter means fortransmitting said trains of pulses in the form of a single train.

10. A multichannel modulator system comprising a, plurality ofmodulators each including means for producing a plurality of separateseries of signal modulated pulses, each series representing a differentsignalling channel, and means for controlling said modulators to spacethe adiacent channels in each modulator a relatively wide interval whilemaintaining relatively close spacing of channels in adjacent modulators.

11. A multichannel modulator system comprising a plurality of modulatorseach including 20 means for producing a plurality of separate series ofsignal modulated pulses, each series representing a different signallingchannel, and means for synchronizing said modulators to interleave theoutput pulses thereof into a single train of pulses.

12. A multichannel modulator system comprising a plurality of modulatorseach including a plurality of separate signal controlled means forproducing separate series of signal modulated pulses, means forpreventing interference between the signal controlled means of eachmodulator, and means for synchronizing said modulators to interleave theoutput pulses thereof into a single train of pulses.

13. A system for translating time displacement modulation signal pulsesinto output pulses whose width varies according to said displacementcomprising, means for producing constant repetition rate control pulsesin synchronism With the signal pulses in their unmodulated state, eachof said control pulses having a width covering the entire range ofdisplacement oi the corresponding signal pulse, a tripping circuithaving two levels of stability, means for biasing said circuit tomaintain it at a rst one of said stability levels, said bias having avalue such that the combined amplitudes of a control. pulse and signalpulse are required to produce tripping to the second stability levelwhile the control pulse alone is suiiicient to maintain the circuit atsaid second level, means for applying the control pulses and the timedisplacement modulated signal pulses to trip said circuit to said secondlevel at the time of application of a signal pulse, and to permit returnto said first level at the end of the corresponding control pulse, andmeans for deriving from said circuit variable width output pulses.

14. A system according to claim 13, wherein said circuit is amultivibrator.

15. A system according to claim 13, wherein said control pulses aresubstantially rectangular.

16. An arrangement for translating time displacement modulation signalpulses into output pulses Whose Width varies according tosaid-displacement, in a communication system in which synchronizingpulses of constant repetition rate are each followed by a signal pulsehaving a time displacement with respect to the associated synchronizingpulse that varies according to instantaneous values of the intelligenceto be conveyed comprising, means for producing under the control of saidsynchronizing pulses control pulses synchronized with said synchronizingpulses and having a Width covering the entire range of displacement ofthe corresponding signal pulse, a tripping circuit having two levels or"stability, means for biasing said circuit to maintain it at a rst one ofsaid stability levels, said bias having a value such that the combinedamplitudes of a control pulse and signal pulse are required to producetripping to the second level while the control pulse alone is suncientto maintain the circuit at said second level, means for applying thecontrol pulses and the time displacement modulated signal pulses to tripsaid circuit to said second level at the time of application of a signalpulse and to permit return to said nrst level at the end of thecorresponding control pulse, and means for deriving from said circuitvariable width output pulses.

17. A system according to claim 16, wherein said tripping circuit isamultivibrator.

18. A system according to claim 16, wherein said control pulses aresubstantially rectangular.

19. An arrangement for translating time displacement modulation signalpulses into output pulses whose width varies according to saiddisplacement and for distributing said output pulses into separatechannels, in a multichannel pulse of each one of the signal pulses, aplurality of tripping circuits each forming a part of a separate channeland each having two levels of stability, means for biasing each of saidcircuits to maintain it at a rst one of said stability levels, said biashaving a value such that the combined amplitudes of a control pulse andsignal pulse are required to produce tripping to the second level Whilethe control pulse alone is suicient to maintain the circuit at saidsecond level, means for applying the control pulses successively to saidtripping circuits to successively prepare them for tripping to saidsecond level during the interval allotted to their correspondingchannel,

means for applying the time displacement modulated signal pulses inparallel to said circuits to trip the corresponding channel circuit tosaid second level upon application of the proper signal pulse, and topermit return to saidY first level at the end of the associated controlpulse, means for deriving from each of said circuits output pulsescorresponding in width to the displacement of the associated signalpulses, and means for applying said output pulses to separate loads'.

20. A system according to claim 19, wherein said control pulses arerectangular.

21. A system according to claim 19, wherein said circuits aremultivibrators.

References Cited in the le of this patent UNITED STATES PATENTS NumberName Date 1,918,252 Dunham July 18, 1933 2,048,081 Riggs July 21, 19352,157,434 Potter May 9, 1939 2,172,354 Blumlein Sept. 12, 1939 2,262,838Deloraine et al Nov. 18, 1941 2,313,906 Wendt Mar. 16, 1943 2,403,210Butement July 2, 1946 2,414,265 Lawson Jan. 14, 1947 2,478,919 HansellAug. 16, 1949 2,478,920 Hansell Aug. 16, 1949

